STAT5 inhibition leads to Suppression of STAT5 binding to cytokine gene promoters

STAT proteins bind with their SH2 domains (which are located between amino acids 600 and 700) to phosphorylated tyrosine residues of transmembrane receptors (Heim, et al. 1995, Stahl, et al. 1995). Once STATs are bound to the receptors, the receptor-associated Jak kinases phosphorylate them on a single tyrosine residue located carboxy terminal of the SH2 domain. Changing this tyrosine to phenylalanine results in STATs that are no longer functional (Shuai, et al. 1993). Two STATs dimerize through specific reciprocal SH2–phosphotyrosine interaction and translocate to the nucleus. After translocation into the nucleus, STATs bind DNA response elements in promoters of target genes. The putative DNA-binding domain lies between amino acids 400 and 500. After DNA binding STATs interact directly or indirectly with the RNA polymerase II complex. The DNA sequence elements in the promoters of genes that bind STAT proteins can be classified in two groups. The prototype of the first class is the interferon-stimulated response element (ISRE).

The second class comprises the GAS-like response elements. STAT5 homodimers have been shown to bind to at least one of the GAS-like elements (Heim 1996).

As a property of STAT, it is known that DNA binding ability is acquired by forming a dimer, and it is considered that a modifying factor does not intervene in that respect.

CD2 signaling of human PBMCs results in activation of the -3.6-kb IFN-γ promoter. In contrast, mutation of the -3.6-kb STAT5 site attenuates promoter activity. Functional activation is accompanied by STAT5A, but scant STAT5B nucleoprotein binds to the STAT5 binding site on the IFN-γ promoter, as determined by competition and supershift assays. Western and fluorescence-activated cell sorting analyses revealed increased phospho-STAT5 following CD2 signaling (Gonsky, et al. 2004).